Friend helper virus (F-eco) is a murine retrovirus which can induce a variety of hematopoietic neoplasms. This virus is being used as a model system to study basic mechanisms of leukemogenesis. 1. Genetic studies. Crosses between AKR and other strains indicated that AKR mice carry at least two genes other than inherited ecotropic viruses which predispose them to lymphoma a few months after inoculation with F-eco. One of these genes may be on chromosome 15, a chromosome which carries an oncogene (myc) and virus integration sites (Mlvi-1 and Mlvi-2) associated with lymphoma. 2. Virological studies. MCF viruses were found in F-eco inoculated mice developing erythroblastosis. However, MCF viruses were not found by standard infectious center assays or blot hybridization techniques in lymphomas or myeloid leukemias indiced by F-eco in C57BL mice. Thus, MCF viruses are not pathogenic intermediates in these neoplasms. When C57BL mice were inoculatd with pseudo-typic mixtures of F-eco plus F-MCF, the MCF viruses grew well but did not cause erythroblastosis. 3. Molecular studies. Virus-cell junction fragments are being cloned from F-eco induced myeloid leukemias in C57BL mice. These experiments are designed to look for common integration sites or cellular oncogenes associated with myeloid leukemias. Significance. Host genes affecting type of leukemia induced by F-eco may elucidate steps in viral leukemogenesis. MCF viruses are not necessry or some types of F-eco induced neoplasms. Common integration sites in myeloid leukemias are being sought as candidate oncogenes involved in myeloid leukemogenesis.